Bedrock
Bedrock is the solid foundation beneath the Earth's surface, comprised of compacted rock that lies underneath layers of looser, weathered materials like soil and alluvium. Typically located hundreds of meters deep, bedrock can sometimes be exposed in areas known as "outcrops," where erosion or weathering has removed the overlying deposits. It is formed through a geological process called lithification, where sediments—particles created from the breakdown of existing rocks—are compacted and cemented over time. This process involves squeezing out trapped fluids and binding sediment grains together through mineral precipitation, resulting in solid rock formations. Bedrock varies in composition, reflecting a range of geological histories, and can hold valuable information about the Earth’s thermal and mineral conditions through trapped connate fluids. Key locations for outcrops include steep hillsides, riverbanks, and areas affected by glaciers or ocean waves. Bedrock is crucial for engineering geology, as it provides stability and is significant in the search for fossil fuel resources, with the interface between bedrock and surface materials referred to as "rockhead." Understanding bedrock is essential for various geological and engineering applications.
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Bedrock
Bedrock is the solid rock that underlies the layers of looser, weathered materials that are deposited on the earth’s surface, such as soil, subsoil, or alluvium. Because these unconsolidated layers can be very thick, the bedrock is often located hundreds of meters below the surface. In some places, however, weathering and erosion have removed surface deposits and exposed the underlying bedrock, or "outcrops." Bedrock is harder and more compact than the surface layers. It need not be homogenous and can consist of many different rocks compressed into a consolidated layer.
Overview
Bedrock is formed through the process of lithification, which comes from ancient Greek words meaning "rock making." In lithification, surface material called "sediment"—any rock that has been broken down into particles, such as by weathering and erosion, and then transported and deposited through the actions of water, wind, or glaciers—turns into solid rock. As layers of sediment pile up, the bottom layers are compacted by the weight of the material above them. The constant pressure gradually compresses the sediment, reducing its porousness.
First, the pressure squeezes out liquids that were trapped in the sediment when it was first formed. These liquids, called "connate fluids," are typically a water-based solution in which mineral components have dissolved. This part of the process can help geochemists understand the historical origins of different layers of bedrock: trapped bubbles of connate fluid act as chemical fingerprints, providing clues about the conditions under which the bedrock formed and the thermal and mineral history of the host rock.
As fluids are squeezed from the pores, the pore spaces decrease, and individual sedimentary grains fuse together into differentiated solid rock. Ions carried in the connate fluids precipitate out and form solid crystalline bridges between grains of sediment, binding them together in a process called cementation. This is the process by which grains of sand are bound together to form sandstone, for instance.
Most cementation occurs below the water table. As a result, the type of cement found in the bedrock on the sea floor is distinct from that in the groundwater zone (the region beneath the earth’s surface where water saturates the spaces left by soil pores and gaps in rock formations). There, cementation is a constant, ongoing process.
Outcrops can form through various processes. In most of the world, vegetation and soil keep deposits in place, not only covering bedrock but replenishing sediment. However, glacial activity can create outcrops, and rocky ocean shores remain so because ocean waves scour outcrops and prevent the buildup of deposits. Steep hillsides, riverbanks where fast-flowing currents cause rapid erosion, and mountain ridges at high elevations with little vegetation are all typical sites for outcrops.
Bedrock is important to engineering geologists both because of its stability and because of its implications in searches for fossil fuel sources. In engineering terminology, the surface of the bedrock where it meets un-lithified deposit layers is called the "rockhead," and the overlying deposit layers are called "drift."
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